It is usually preferable that polyester (or copolyester) pellets be produced and supplied to customers/processors in a crystalline form. This is because they can be dried at higher temperatures (without sticking together) and because they will feed better down the barrel of an extruder or injection molding machine. Furthermore, having crystalline pellets is advantageous from a manufacturing standpoint in that, optionally, they can be further polymerized (without melting) via a process known as "solid stating". Solid stating is more economical than "melt phase" polymerization and also has the additional benefit of removing undesirable volatiles like acetaldehyde.
Typical "melt phase" polyester reactors produce only amorphous pellets. To make these pellets crystalline, they are usually heated to elevated temperatures, while being constantly stirred/agitated in some manner in order to prevent sticking or clumping. This process, which we will refer to as "thermal crystallization" to distinguish it from the current invention, is usually performed in a "crystallizer". The crystallizer is nothing more that a heated vessel with a series of paddles or agitator blades to keep the pellets stirred (e.g. a Bepex solid stater). Alternately, a crystallizer can consist of a hot, fluidized bed for keeping the pellets apart (e.g. a Buhler solid stater). If the polyester or co-polyester crystallizes very slowly, then the latter type cannot be applied because the softened sticky pellets will eventually clump together and disrupt the fluidized bed before crystallization can occur.
A typical PET manufacturing process involves forming the polymer via melt phase polymerizing up to a certain IV or molecular weight, extruding and cutting the polymer strands into amorphous pellets, crystallizing, and then heating to an even higher temperature while under nitrogen purge (or vacuum) in order to continue to build molecular weight or IV (i.e. solid stating). Unfortunately, the solid stating process can not be applied directly to amorphous polyesters because they will flow and stick together. Thus, most hard-to-crystallize polyesters are not solid-stated.
The ability to rapidly crystallize amorphous pellets is not only beneficial for manufacture of the resins (via solid stating), but, as mentioned before, is also of benefit to the extruder/processor since it allows for higher drying temperatures. Polyesters are hydrolytically unstable and have to be thoroughly dried before extruding or molding to prevent IV degradation. Being able to dry at higher temperatures means better drying efficiency. Amorphous polyesters can only be dried at temperatures below the Tg of the polymer (typically 70 to 80.degree. C.) because of the sticking/clumping problem. Crystalline versions of the same polyesters, however, can be dried at much higher temperatures (usually around 150 to 175.degree. C.) and thus can be thoroughly dried in a much shorter time.
The traditional thermal crystallization methods (i.e. a standard "crystallizer") have a number of problems. First, the agitation required to keep the pellets from sticking can also cause chipping and other damage to the pellets; which can lead to dust generation or the presence of "fines" in the crystallizer. These small pieces of chipped off plastic can often cause extrusion problems if not properly filtered. Another serious problem is that thermal crystallization methods do not usually work well for co-polyesters. Co-polyesters crystallize at a much slower rate than homopolymers. Thus they may require excessively long residence times in a thermal crystallizer in order to achieve an acceptable level of crystallinity.
Thus there remains a need in the art for improved methods to crystallize polymers having high I.V., such as those which are solid stated.
JP 07223221 and JP 08155952 and involve the preparation of pellets by stretching solidified strands or sheet. The polyesters described have low melting points (170.degree. C. or less) and glass transition temperatures up to 40.degree. C. and are prone to aggregation during storage or drying.
U.S. Pat. No. 5,292,865, focuses primarily on the after-treatment of polyester pellets to improve devolatilization of acetaldehyde and solid state polycondensation. U.S. Pat. No. 5,292,865 also discloses a process in which polyester strands can be cooled after exiting the reactor die, stretched at least 2.times. and then immediately chopped to form pellets. The stretch temperature is between Tg and 30.degree. C. below the melting point. However, U.S. Pat. No. 5,292,865 does not disclose that the strain induced crystallization will be sufficient to impart crystallinity to otherwise amorphous or difficult to crystallize polyesters, or that those polyesters could then be solid stated without a thermal crystallization step.
WO 98/02479 discloses a compositions having 5-25 weight % isopthalic acid. The compositions are formed into article such as film, fibers and containers which are crystallized using both strain and thermal induced crystallization. WO 98/02479 does not disclose or suggest imparting strain induced crystallization prior to pelletization.
Processes for rapidly crystallizing a polymer strand by stretching/drafting are commonly used in the fiber and monofilament industry. However, the fiber/monofilament "melt to spin" processes produce final strands having significantly different final strand diameters and lengths as well as improved tensile and mechanical properties which remain with the strands through use because the strands are not re-melting.